Fertilization is the result of a set of well-choreographed events between the sperm and the egg. Successful gamete interactions result in the activation of the egg and initiate developmental pathways. Unsuccessful interactions, as a result of either sperm or egg dysfunction, are disastrous. Although much is known about the steps preceding binding and sperm-egg fusion, the mechanism of the binding event and the resulting signal transduction pathways that occur within the egg have been elusive. The recent identification and cloning of the sea urchin egg plasma membrane receptor for sperm now makes it possible to investigate these questions and to determine how the receptor functions at egg activation. This proposal describes experiments designed to address this structure/function question in the sea urchin, a well-characterized model system for fertilization and early development. The general question centers on the role that the receptor plays in transducing a signal in response to sperm binding, be it direct or indirect. Our preliminary data indicate that the receptor interacts with an egg cortical protein tyrosine kinase and may serve to organize the actin cytoskeletal rearrangements that occur at fertilization. The specific aims are: (i) To define the topology of the receptor in the membrane in order to establish which domains may interact with other egg proteins; (ii) To identify egg proteins that interact with the receptor; (iii) To test the functional role(s) of the cytoplasmic domain by injection of specific antibodies and peptides; and (iv) To initiate mutational analyses of the receptor, employing a heterologous expression system. These experiments will provide for a greater understanding of how sperm binding triggers the changes that occur in the egg, adding to our general knowledge of the basic processes of fertilization so important to our understanding of fertility and infertility.